Previously, the patent literature 1 discloses a pump as a technique that is applicable in a fuel pump, which suctions fuel and discharges the suctioned fuel. This pump has: an outer gear, which includes a plurality of internal teeth; an inner gear, which includes a plurality of external teeth and is meshed with the outer gear while the inner gear is eccentric to the outer gear in an eccentric direction; and a pump housing, which rotatably receives the outer gear and the inner gear. When the outer gear and the inner gear are rotated to increase and decrease volumes of a plurality of pump chambers, which are formed between the outer gear and the inner gear, fuel is sequentially drawn into and is discharged from the pump chambers.
The pump housing includes: a pair of slide surfaces, which hold the outer gear and the inner gear from two opposite sides, respectively, in an axial direction, so that the outer gear and the inner gear are slid along the pair of slide surfaces; a suction guide passage that is recessed from the slide surface and guides liquid at a suction side; and a discharge guide passage that is recessed from the slide surface and guides the liquid at a discharge side.
Furthermore, the pump housing includes a pressure drain passage that is shaped into a linear form and communicates between the suction guide passage and the discharge guide passage. The pressure drain passage limits application of an excess load to the electric motor that is caused by exertion of a pressure, which is larger than a discharge capacity of the fuel pump.
The fuel pump may possibly suction foreign objects contained in the fuel. In the pump housing, at tooth tips of the external teeth of the inner gear, the inner gear and the outer gear can be brought close to each other, and thereby a density of the foreign objects can become particularly high. The foreign objects, which are present at the proximity location where the inner gear and the outer gear are brought into close proximity to each other, may possibly be slid along the slide surface in an area where a relief path, such as a guide passage, is absent. The inventors of the present application have found that slide scratches are generated at the slide surface along a circumcircle of the inner gear due to the sliding of the foreign objects such that a depth of the slide scratches is progressively deepened through use of the fuel pump. A pump efficiency may be deteriorated due to fuel leakage from the discharge guide passage to the suction guide passage.
The pressure drain groove of the patent literature 1, which is shaped into the linear form, may possibly enable relief of the foreign objects of the proximity location at a location where the circumcircle of the inner gear overlaps with the pressure drain groove. However, at a location, at which the circumcircle of the inner gear does not overlap with the pressure drain groove, the foreign objects of the proximity location are slid along the slide surface to cause generation of the slide scratches. In contrast, in a case where a width of the pressure drain groove, which is shaped into the linear form, is increased to cover all of the circumcircle, the suction guide passage and the discharge guide passage are substantially connected together to significantly deteriorate the pump efficiency.